1. Field of the Invention
The present invention relates to a solid-state image sensing apparatus, method for driving the same apparatus and camera and more particularly to a so-called wide dynamic range CCD (Charge Coupled Device) solid-state image sensing apparatus assuring a signal output of wide dynamic range for an optical input, a method for driving the same apparatus and a camera using, as an image sensing device, a CCD solid-state image sensing apparatus of wide dynamic range.
2. Description of the Related Art
In a CCD solid-state image sensing apparatus, each pixel (photoelectric conversion element) conducts photoelectric conversion, a signal output becomes constant after the accumulated signal charge overflows from the pixel and therefore the signal output for the amount of further incident light can no longer be obtained. Therefore, dynamic range for incident light is rather narrow and when a person standing at the side of window, for example, is picked up by a camera from the inside of room, such person is picked up as a black image.
In view of widening the dynamic range, following method has been introduced in the related art, in which after the signal charge based on the ordinary exposing time in the effective period in the vertical direction is read, a short exposing period is set again within the vertical blanking period, the signal charge based on this short exposing time is read again, the signal charge of longer exposing time and the signal charge of shorter exposing time are respectively converted to electric signals in the charge detecting section and these signal charges are generally guided as the exposed signal and blanking exposed signal and are then added in a line memory of the external signal processing system.
However, this method of related art has a problem that since an ordinary exposed signal or blanking exposed signal of 1H (H: horizontal scanning period) is required to be once stored in the line memory to add both exposed signals, a line memory and a control circuit are required for the external signal processing system of the solid-state image sensing apparatus and thereby the circuit structure of the relevant signal processing system is complicated.
Meanwhile, there is provided a CCD solid-state image sensing apparatus (refer to Japanese Published Unexamined Patent Application No. HEI 4-298175) for which a group of pixels is formed by a plurality of neighboring pixels arranged in two dimensions, the pixels in each group are given different photoelectric sensing characteristics, signal charges of the pixels in different photoelectric sensing characteristics are individually read and are then transferred to the charge detecting section by the vertical and horizontal transfer sections, the signal charges in higher photoelectric sensing characteristics are clipped by the predetermined clip level in the relevant charge detecting section and the signal charges in the lower photoelectric sensing characteristics are added to the clipped signal charges in the higher photoelectric sensing characteristics.
As explained above, in the CCD solid-state image sensing apparatus of the related art in which the clipping and addition are conducted within the device, it is not required to provide the line memory and the control circuit in the external signal processing system. Therefore this apparatus has a merit that the circuit structure of the relevant signal processing system can be simplified but also has the following new problem because of the clipping and addition of the signal charges.
Namely, in the charge detecting section consisting of an FD (floating diffusion) area for converting, as an output, the signal charge to a signal voltage, an RD (reset drain) area for exhausting signal charge in the FD area and a reset gate section having single reset gate electrode provided between the FD area and RD area, the predetermined clip level is set first to the low level of the reset pulse to be applied to the reset gate electrode, the signal charge of higher photoelectric sensing characteristic is clipped by this clip level, the lower level of the reset pulse is set to the further low predetermined level, the signal charge of lower photoelectric sensing characteristic is added to the clipped signal charge of the higher photoelectric sensing characteristic, thereafter the low level of the reset pulse is set to the reset level for the reset operation. Accordingly, the low level of the reset pulse must be switched in the three stages.
As explained above, for the clipping and addition in the charge detecting section, the pulse taking the three levels must be used as the reset pulse and the circuit structure of a pulse generating circuit to generate such reset pulse is complicated and the timing control becomes very difficult. Particularly, since the reset period of the reset gate section is as high as xc2xd the drive frequency of the horizontal transfer section, the higher rate is also required for the timing control of the reset pulse taking the three levels. However, here rises a problem that it is very difficult to realize such high speed timing control for the reset pulse.
The present invention has been proposed considering the problems of the related art explained above and it is therefore an object of the present invention to provide a solid-stage image sensing apparatus, the method for driving the same apparatus and the camera which can realize the clipping and addition of the signal charges within the device through the simplified timing control.
The solid-state image sensing apparatus of the present invention has a structure of comprising the charge detecting section consisting of an output diffusion area for detecting, as an output, the signal charge transferred from the image sensing section to an electric signal, a reset drain area for exhausting signal charge in the output diffusion area and a reset gate section having a plurality of reset gate electrodes arranged in vertical between the output diffusion area and reset drain area.
In the charge detecting section of the structure explained above, since the reset gate section has a plurality of reset gate electrodes, a plurality of reset pulses can be applied to a plurality of reset gate electrodes. By setting the low level of these reset pulses to an adequate value, a certain reset electrode performs the clip operation and reset operation for the signal charge transferred to the output diffusion area, while the other reset gate electrodes execute addition and reset operation.
Moreover, the method for driving the solid-state image sensing apparatus of the present invention comprises the steps, in the solid-state image sensing apparatus including an image sensing section for sequentially obtaining a plurality of kinds of signal charges in different photoelectric sensing characteristic and a charge detecting section of the structure explained above, that a first reset pulse is applied at least to the reset gate electrodes other than the reset gate electrode nearest to the reset drain area among a plurality of reset gate electrodes, the signal charges of higher photoelectric sensing characteristics (hereinafter, referred to as high sensitivity signal charges) in the output diffusion area are clipped by the clip level determined by the low level of the first reset pulse, the second reset pulse of the level lower than the first reset pulse is applied, after the clipping, to the reset gate electrode in the reset drain area side, the signal charges of lower photoelectric sensing characteristics (hereinafter referred to as low sensitivity signal charges) are added to the clipped high sensitivity signal charges, and thereafter high level of the first and second reset pulses is set as the reset level.
In the solid-state image sensing apparatus of the structure explained above, the first reset pulse of which low level is higher than the reset level is previously impressed first at least to the reset gate electrode other than the reset gate electrode nearest to the reset drain area among a plurality of reset gate electrodes. Under this condition, when the high sensitivity signal charge is transferred to the output diffusion area, this high sensitivity signal charge is clipped by the clip level determined by the low level of the first reset pulse. With this clipping operation, even if the incident light is applied to the pixel in such amount as saturating the pixel having the high photoelectric sensing characteristic, fluctuation of the overflow characteristic of each pixel can be eliminated.
Upon completion of this clipping operation, the second reset pulse of which low level is lower than the first reset pulse is impressed to the reset gate electrode in the reset drain area side. In this case, the clipped high sensitivity signal charge is being accumulated in the output diffusion area. Under this condition, when the low sensitivity signal charge is transferred to the output diffusion area, the clipped high sensitivity signal charge and low sensitivity signal charge are added within the output diffusion area. With this adding operation, the pixel signal can be obtained and thereby the dynamic range can also be widen. The reset operation can also be conducted by setting the high level of the first and second reset pulses to the reset level.
The camera of the present invention uses the solid-state image sensing apparatus of the structure explained above as the image sensing device. In this case, image sensing of the wider dynamic range can be realized. For example, even when a person standing by the side of window is picked up from the side of room, a person will never be picked up as a black image and good image can be picked up. Particularly when the structure allows selection of the addition mode for adding the signal charges in the output diffusion area of the charge detecting section and the no-addition mode, image can be picked up in the ordinary mode in addition to the image pickup in the mode for expanding the dynamic range.